Resellers can access their clients' control panel to make changes simply by substituting the user's password with their own primary WHM account password.
This facility will grant you access to most areas of the client's control panel, with the exception of being able to access any database data related utilities, such as backups of phpMyAdmin.
Once logged in as a client, you can also switch to other customer accounts by using the drop-down box situated at the top of the control panel page.